Actually the GPU only works if it has been initialized by fglrx and then the
driver is switched to radeon without rebooting. When booting with radeon it
doesn't work.

I bruteforced all 24 tiling configs by setting them in
evergreen_interpret_tiling  in radeonsi_pipe.c and none of them worked for
eglgears_screen. They all lead to the same result. The tiling config given to
evergreen_interpret_tiling is 0x1023. I have no idea what the (1<<12) bit in it
means.